Combustion Flame Research Articles

LES investigation of a H2-fueled supersonic combustor: A two-strut injection approach

Efficient combustion schemes are increasingly vital as scramjet expansion into higher Mach numbers emerges as a leading trend. Thus, in the current work, an LES investigation on mixing improvement caused by a two-strut injector has been carried out for a Mach 2.5 model H2 fueled scramjet combustor. The research focuses on understanding the flow field, flame lift-off characteristics and combustion stabilization in the two-strut combustor. The simulation results indicate that the two-strut configuration exhibits a broader pattern of temperature fluctuation, implying a more efficient spread of combustion downstream compared to the single-strut configuration. Analysis of Fast Fourier Transform (FFT) pressure data reveals inherent instability in the combustion processes for two-strut under supersonic air inflow conditions. The findings also demonstrate the significant improvement in mixing performance achieved by the two-strut design compared to conventional struts. The primary mechanism is a two-strut's ability to create significant streamwise vorticity, which can improve the mixing of H2 fuel with supersonic air. According to the mixing and total pressure loss plots, a two-strut produces a rapid complete mixing at 0.182 m at the expense of a minute rise in total pressure loss. Finally, the entropy change plot reveals that the two-strut configuration exhibits a reduced entropy change compared to the single-strut configuration. This is mainly due to enhanced air-fuel mixing, efficient shock wave management, and enhanced flame stabilization in the two-strut setup. These factors collectively contribute to reduced irreversibilities and disorder in the flow, leading to lower entropy within the system.

Ultralean Combustion Mechanism of Methane–Air Swirling Premixed Flame

ABSTRACT This study investigated a methane – air premixed flame in a partially tapered swirl burner. Experimentally, we succeeded in forming almost steady ultralean flames with an equivalence ratio as lean as 0.465. On the other hand, a numerical simulation of the flame with detailed chemistry revealed that the flame head existed in a large recirculation zone under ultralean conditions. In addition, high-value peaks of temperature, heat-release rate (HRR), and concentrations of OH, O, and H radicals were formed at the flame head, enabling ultralean combustion. Subsequently, to identify the formation mechanism of these peaks, adiabatic swirl burner flames were simulated with and without the effect of thermal-diffusive imbalance (a Lewis number effect). The results show that a Lewis number of less than unity caused a temperature increase in the flame located in the recirculation zone owing to the thermal-diffusive imbalance, which increased the peak heights of the HRR and the three radicals. The results also showed that even in a flame unaffected by thermal-diffusive imbalance, the backward convective transport could accumulate CO and OH at the flame head thus enhancing the chemical reactions at the head. (185 words)

Expanding the Capability of a Legacy Combustion Flame Tube to Test High-Temperature Engine Materials in Relevant Environments

Abstract New materials and component designs are needed to advance gas turbine engine technology and provide the performance and efficiency needs for future applications. In order to advance these materials, testing in combustion environments is a critical step prior to engine testing. In this work, we detail the design and the fabrication of a materials test sector in a flame tube combustor facility. The facility simulates a combustion environment similar to that experienced by components in gas turbine engines. The flow regime is characterized by a combination of high-temperature, high-heat flux, and high-velocity that components experience in gas turbine engines. Exposure of components in this facility allows for the study of combined environmental effects and the impact on both coating and substrate durability. The test facility may operate across a wide range of pressures from 275 to 400 psig (1896–2758 kPa) and an air flowrate of 5 lb/s (2.27 kg/s). While combustion gas temperature is expected in excess of 3000 °F (1649 °C), 900 °F (482 °C) cooling air may be supplied to the backside of components or test articles. The flame tube combustor was previously used to evaluate fuel injectors and combustion products, and the new test configuration will also allow for materials exposure to complex, engine-like conditions. The interior of the test section was additively manufactured from GRCop-84 and cryogenically fit and brazed to a stainless-steel 304 housing. The use of a copper liner minimizes welds and, with active cooling, is expected to provide better durability over traditional hardware using stainless-steel or Inconel with a ceramic liner. The test section has two opposing 9.5 in × 3.125 in (241 mm × 80 mm) removable windows that can accommodate articles up to 3.5 in (89 mm) tall. This modular design allows for custom platforms to hold coupons, panels, or airfoil shapes to be tested with minimal re-engineering or fabrication. The bolted joint and sealing remains consistent, so any new testing only needs to work within the existing design footprint. This paper will provide an overview of the facility capabilities, design considerations, as well as thermal and structural analysis of the hardware. Future testing of ceramic matrix composite (CMC) airfoils and advanced environmental barrier coatings (EBCs) will also be discussed.

Ear Reconstruction for Auricle Defects Using an Expanded Neck Flap: A Case Report.

This case report describes a patient with a left ear deformity resulting from a flame burn sustained 20 years ago. The patient underwent an ear reconstruction procedure utilizing an expanded neck flap. The autologous rib cartilage was used as the framework, while an expanded neck flap served as the covering for the framework. The surgery was completed in 3 stages. Initially, tissue expanders were implanted and gradually inflated with water. After sufficient expansion, the expanders were removed, and the scar tissue was excised. Subsequently, the expanded flap was used to cover the defects, and the expanded neck flap was rotated to cover the autogenous costal cartilage framework obtained intraoperatively. Finally, the reconstructed ear was repaired by constructing the cephaloauricular sulcus, removing postauricular scars, and trimming the neck-flap pedicle. After a 1-year follow-up, the wound had healed satisfactorily with only minor complications. The shape of the reconstructed ear appeared realistic, and its function was maintained. Most of the scars were repaired, and the scarred alopecia was significantly improved. In patients with limited availability of the postauricular flap, especially burn patients, using an expanded neck flap can lead to superior outcomes.

Effect of Pilates exercises on pulmonary function, respiratory muscle strength, and functional capacity in patients with inhalation injury after flame thermal burn: A prospective randomized controlled trial

BackgroundInhalation injury is an acute respiratory tract injury that occurrs by inhalation of smoke, toxic gases, or steam. Early management is needed to reduce its mortality and morbidity. The purpose of this study was to ascertain whether Pilates training could help burn patients with inhalation injury after flame thermal burn in increasing pulmonary function, respiratory muscle strength, and functional ability. MethodsIn this prospective, randomized, controlled trial, sixty participants (26 males and 34 females) with inhalation injury and deep partial-thickness flame burns of 30–40 % total body surface area (TBSA) were randomized in blocks of four, with a 1:1 allocation ratio into two groups: Group A (Pilates Group); received Pilates training plus conventional physical therapy program, and Group B (Control Group); received conventional physical therapy program only. This study was conducted at the Faculty of Physical Therapy's outpatient clinic, Cairo University, 3 sessions/week for 12 weeks. The primary outcome measure was the forced vital capacity (FVC) measured by a spirometer, while the secondary outcome measures were peak expiratory flow rate (PEFR), forced expiratory volume in 1 s (FEV1), FEV1/FVC% assessed by a spirometer, strength of respiratory muscles (maximum inspiratory pressure (MIP) and maximum expiratory pressure (MEP) assessed by the digital manovacuometer, and the functional capacity evaluated by 6-Minute Walk Test (6-MWT). ResultsA two-way mixed-design MANOVA was used to analyze the results within and between groups. There were no significant differences in demographic data between groups (P > 0.05). There were significant differences in all variables after treatment in group A compared with group B; FVC (95 % CI: 0.38, 1.13) (P = 0.001), FEV1 (95 % CI: 0.39, 0.97) (P = 0.001), FEV1/FVC % (95 % CI: 1.90, 17.19) (P = 0.02), PEFR (95 % CI: 0.47, 0.99) (P = 0.001), MIP (95 % CI: 5.12, 11.44) (P = 0.001), MEP (95 % CI: 2.57, 8.24) (P = 0.001), 6-MWT (95 % CI: 27.22, 54.96) (P = 0.001), FVC (% predicted) (95 % CI: 3.58, 12.58) (P = 0.001), FEV1 (% predicted) (95 % CI: 1.21, 11.11) (P = 0.02), PEFR (% predicted) (95 % CI: 1.33, 10.83) (P = 0.01), MIP (% predicted) (95 % CI: 2.26, 11.72) (P = 0.001) and MEP (% predicted) (95 % CI: 1.33, 10.37) (P = 0.01). ConclusionThe current study demonstrated that a Pilates exercise program in addition to a traditional physical therapy program for 12 weeks significantly improved the pulmonary function (FVC, FEV1, PEFR and FEV1/FEV), strength of respiratory muscles (MIP and MEP), and functional capacity (6-MWT) in patients with inhalation injury after flame burns. These results underscore the importance of including Pilates exercises in the rehabilitation plan for burn patients with inhalation injury. Future studies are needed to evaluate the effect of Pilates exercises on other degrees and TBSA of burn, long-term follow up, and to measure cardiopulmonary parameters.

Fire and smoke transport dynamics in a dead-end tunnel under heavy rainfall

Fire is one of the most common major accidents during tunnel construction and operation. This work conducted scale-model experiments to investigate the pool burning progress and flame characteristics in a dead-end tunnel with one end closed and the other open under heavy rainfall. The results show that, compared to opening both ends, the global burning time of fires is significantly decreased, and the quasi-stable burning phase is advanced under the combined effects of the closed end and heavy rainfall. The reason is that the increase in ceiling temperature, resulting from the accumulation of hot smoke, leads to an increase in radiative feedback of the fuel from the environment and an acceleration in the pool's burning rate. A coefficient (denoted by γ) is defined to evaluate the increase in the burning rate resulting from the dead-end. The enhancement ratio is found to be related to rainfall intensity, raindrop size, and pool size, and a correlation has been established based on the experimental results. The fire has an increased flame height and a reduced tilt angle under the impact of the dead-end. The changes in flame geometry parameters are mainly influenced by thermal buoyancy and the restriction of the dead-end. Prediction formulas for flame height and tilt angle under the combined effects of the dead-end and heavy rainfall have been proposed based on theoretical analysis. Under these combined effects, the tunnel space becomes eroded by dirty air.

Numerical simulation of natural gas-ammonia combustion characteristics in a U-shaped radiant tube

NH3, recognized as a carbon-free fuel and a high-energy–density renewable hydrogen carrier, holds promise as a low-carbon alternative fuel. However, its direct combustion is often posed with challenges such as a limited ignition zone and high NOX emissions. To address this, the present paper proposes a strategy to blend more reactive hydrogen-containing fuels with ammonia to enhance combustion stability and concurrently reduce NOX emissions. This study developed numerical models for natural gas-ammonia combustion within a U-shaped radiant tube. The dynamic characteristics of combustion flame flow fields inside the radiation tube were investigated across various mixing ratios, analyzing the effects of nitrogen-containing fuel temperature on NOX generation and elucidating NOX generation patterns under different mixing ratios. The findings indicated that a 30 % NH3 blending ratio in the radiation tube yielded uniform wall temperature distribution along its length, minimal circumferential temperature variation, and an outlet NOX emission of only 20.3 ppm. Furthermore, Pearson correlation coefficient calculations revealed strong correlations between NH3 concentration (0.97), URT temperature (0.88), and NOX generation rate (0.88) within the U-type radiation tube combustion flame and NOX emission content.

Justification of the Efficiency of Application of Plaster for Fire Protection of Concrete Structures

The problem of using concrete for building structures is to ensure their stability and durability during operation within wide limits. Therefore, the object of research was the change in the properties of concrete during a fire and its protection when applying a heat-insulating layer of plaster, which is able to inhibit the temperature when the flame affects the coating. It has been proven that in the process of thermal action on fire-resistant plaster, the process of thermal insulation of concrete consists in the application of materials with low thermal conductivity as part of the plaster on the surface of the material. Similarly, under the influence of the burner flame, a temperature was created on the surface of the sample under the influence of the burner flame with a temperature of more than 800°С for 1800 s, the temperature on the surface of the concrete under the plaster coating did not exceed 120°С, which indicates the formation of a curtain for temperature. In this regard, experimental studies were conducted and it was established that the presence of aluminosilicate microspheres in the plaster leads to the formation of a thermally protective layer on the surface of concrete resistant to mechanical vibrations. According to experimental data, the coefficient of thermal conductivity and thermal conductivity of the plaster was calculated, which is 6.22·10–7 m2/s and 0.142 W/(m∙K), respectively, due to the addition of aluminosilicate microspheres. Thus, there are reasons to assert the possibility of targeted regulation of concrete fire protection processes by using fire-resistant plaster capable of forming a protective layer on the surface of the material, which slows down the rate of heat transfer.

Controllable generation of engine exhaust similar soot particles using an inverted nitrogen-diluted flame burner for calibration purpose

The calibration of vehicle engine exhaust PN analyzers has been a challenge due to the significant difference in properties between calibration particles and measured particles. In this study, an inverted nitrogen-diluted flame burner was designed to generate engine exhaust similar soot particles and evaluated for calibration use. The burner can produce reference-unimodal soot particles at 61–311 nm and total number concentration up to 108 #/cm3 by adjusting the flow rate of ethylene, nitrogen and air. Excellent stability of 0.91 % enables it to generate a fairly stable calibration particle flow. Mixing nitrogen in ethylene makes burner have a high degree of variability in generating smaller particles. The properties of size distribution, morphology, effective density indicate that the generated soot particles can simulate well with engine exhaust particles. TOA results indicate that the soot have high TC mass concentration of over 559.26 mg/m3 and EC/TC fraction of over 90 %.

Role of Free Flaps in Facial Burn Reconstructions

AbstractTraditionally, burn reconstructions have been performed by the use of skin grafting or local flaps. Recently free flaps are being used with increasing frequency. Although not very common in the head neck region, free flaps are mostly used for secondary reconstructions of cervicofacial contractures. We report the role of free flaps in postburn facial reconstructions, excluding neck and scalp burns.Sixteen free flaps used for postburn facial reconstructions were reviewed retrospectively, during the period between 2003 and 2023. The etiology, indications, timing, location, choice of the flap, type of reconstruction, and outcomes were analyzed. Indications and type of reconstructions were categorized to correlate with flap choice.The age of the patients ranged between 8 and 40 years. The etiology included electrical burns in six cases, flame burns in eight cases, and acid burns in two cases. Nine defects were in the central part of the face including the nose and the chin. Two primary and 14 secondary reconstructions were performed using free flaps from lateral thigh in 11 cases, lateral arm in 2 free flaps cases, radial forearm in 2 free flaps cases, and the posterior auricular flap in 1 case. There were no total flap failures. Secondary procedures were needed in 10 of 13 evaluable patients.Free flaps provide a good and safe option for selective postburn reconstructions in the face. The choice of flap mainly depends on the indication and type of reconstruction needed, apart from the availability of donor tissue and the surgeon's preference. Complex reconstructions may need larger and composite flaps to replace the components. Multiple secondary procedures are needed to achieve the objectives.

Экспериментальные исследования формирования и горения водородно-воздушных смесей при проливах жидкого водорода и при воздействии на горение смесей водяных завес в частично ограниченном пространстве

Experimental studies of processes of liquid hydrogen spill and evaporation and the generation of explosive and fire-hazardous hydrogen-air mixtures in a partially confined space with varying degrees of confinement (varying degrees of leakage of openings) on a stand of 96 m3 volume have been conducted, which involved the ignition of mixtures with delayed and non-delayed inflammation and the measurement of overpressure of explosion; the impact of water screen on these processes have been studied as well. The experiments with duty torches feature rapid combustion of the mixture when the boundary of the generating explosive cloud approaches the area of installed burners, and the volume of the faintly glowing flame of the burning-out mixture rises to a significant height. For all the experiments, the overpressure in the shock wave generated by the flame was approximately the same, equal to 0.8–1.2 kPa. For experiments with delayed ignition, the generation and development of a visible hydrogen-air mixture cloud and its rapid combustion at the activation of burner flares were registered; the visible flame velocities reached 3,050 m/s. At the same time, the delayed ignition within the range of 1–3.2 s did not cause a significant increase of overpressure in the shock wave. Therefore, the specifics of the mixture combustion process in case of liquid hydrogen spills in a confined space are mainly determined by the conditions of mixture generation that significantly differ from the conditions of the generation of hydrogen-air mixture in free space. When hydrogen-air mixtures are exposed to a water screen after the ignition in a confined space, as well as in the air atmosphere, the processes of mixture combustion are activated due to initial combustible mixture flame turbulence. The turbulence of the initial combustible mixture flame is conducted on droplets acting as a periodic volumetric obstacle on the route of the flame front intensifying the flame. At the same time, water screens, with preemptive action, reduce the pressure in the shock wave by the time of the hydrogen spill.

PAM material that instantly gives ordinary fabrics excellent flame retardant and thermal insulation properties for fire rescue

To effectively reduce the damage caused by flame burns or heat transfer to the human body during fire, we used PAM aqueous solution as the matrix, XG as the thickener, HPMC as the water-retaining agent to form the basic material system, and added different functional particles (APP, PTW, HCB) to prepare a fire-proof and heat-insulating PAM flame-retardant material for fire emergency rescue. Ordinary cotton fabrics were impregnated into PAM flame-retardant materials using a simple impregnation process. After the impregnation, the test was performed in a non-dropping state (simulating the thermal protection effect of PAM flame retardant materials directly acting on the outside of the human body at the fire scene). The results show that the PAM flame retardant material prepared by adding 4 wt% HCB has the best comprehensive performance. TPP test shows that spraying PAM flame retardant material on the outside of the fabric can instantly give the fabric a higher thermal protection performance. Under the total heat flux of 84 kW/m2, the thermal performance protection value of the fabric is 2641.8 kW·s/m2, and the second-degree burn time can reach 31.45 s. PAM flame retardant material does not damage the fabric. After soaping, the air permeability of the fabric decreases slightly, the moisture permeability and wettability are improved, and the breaking strength is almost unchanged. The CCT test showed that the thermal radiation flux was 50 KW/m2, the PHRR value of PAM flame retardant material was 10.64552 KW/m2, the THR was 6.9 MJ/m2, and the flame retardant performance was excellent. The PAM flame retardant material prepared in this project can be applied to the fire scene and directly sprayed on the outside of the clothing of rescuers and recipients, giving the fabric a better thermal protection effect. It can also be used to extinguish fires in the external environment. This material offers a novel solution for enhancing fire rescuers' and victims' safety protection levels.

First applications of ultrasound technology in solid rocket propellant combustion promotion

The regulation of propellant combustion using ultrasonic waves is proposed. Under ultrasonic frequencies of 25–40 kHz, we systematically examined the combustion characteristics of Al particles, ammonium perchlorate (AP)/hydroxyl-terminated polybutadiene (HTPB) propellants, and Al-containing Al/AP/HTPB propellants. Ultrasonic treatment increased the ignition delay time of the Al particles by 48.3 %. However, it increased the burning rate of the AP/HTPB propellants by up to 26.1 % and decreased their ignition delay by 39.3 %. As the ultrasonic frequency increased, the burning rate of the solid Al/AP/HTPB propellants increased by 22.5 %, and the degree of Al agglomeration decreased, resulting in a 24 % decrease in the size of the condensed-phase combustion products. The action mechanism was analyzed in terms of the effects of ultrasonic waves on Al droplets and combustion flames. The introduction of ultrasonic waves split the Al droplets near the burning surface into smaller particles, which affected combustion efficiency. Bringing the diffusion flame closer to the burning surface affected the burning rate. These findings demonstrate that the combustion and agglomeration characteristics of solid propellants can be modified using ultrasonic techniques, providing a new method for controlling the thrust of solid rocket motors.

Calibration techniques for quantitative NO measurement using Laser-Induced Fluorescence

Laser-Induced Fluorescence (LIF) is an essential optical diagnostic technique for the high-resolution and low-uncertainty measurement of combustion species concentration in a variety of applications and conditions. Two different calibration techniques are explored in this study to obtain quantitative Nitric Oxide (NO) concentration measurements in flames. The first technique, the most employed in the literature, uses the extrapolation of the fluorescence signal from seeded to nascent NO and is only valid under negligible NO reburn conditions. The second technique uses the optical calibration of the experimental setup to relate it to a modelled LIF signal and can be applied regardless of NO reburn. Both of these techniques are explored under two different assumptions: constant and non-constant interfering LIF signal on the NO absorption spectrum. While the former is most often used in the literature, the latter is necessary when the LIF signal from interfering species cannot be distinguished from the NO-LIF signal, especially in high pressure conditions. Hence, a total of four techniques are presented in this work and are found to be in excellent agreement when performed in different flame conditions. The calibration techniques are applied to three lean, atmospheric, laminar, premixed, methane-air flames to explore their field of applicability. Specifically, the study explores the relevance of the techniques in reburn conditions, which occur mostly in high pressure, rich, highly-seeded, or NH3-containing flames. This study aims to offer the reader a portfolio of calibration techniques to use according to the conditions in which they need to be applied. While this study was carried out measuring NO concentration in a stagnation flame burner, the concepts and equations presented can be transposed to the measurement of other species and to other experimental configurations.

Three-dimensional particulate volume fraction reconstruction in the fluid based on the Lambert–Beer physics information neural network

The measurement of particle volume fraction in flow fields is of great significance in scientific research and engineering applications. As one of the particle detection techniques, the light extinction method is widely used in measuring nano-particles volume fraction in flow fields due to its simplicity and non-contact nature. In particular, in complex reactive flow fields like combustion reactions, the volume fraction of soot particulate and other particles can be accurately measured and reconstructed via the light extinction method that based on the Beer–Lambert law. This is crucial for exploring combustion phenomena, understanding their internal mechanisms, and reducing pollutant emissions. However, due to the enormous computational burden, current algebra reconstruction techniques struggle to achieve high-precision three-dimensional (3D) reconstruction of particles. Therefore, this paper originally proposes a 3D reconstruction algorithm based on the Beer–Lambert law physical information neural networks (LB-PINNs). By incorporating physical information as constraints into the particle reconstruction process, it is possible to achieve high-precision 3D reconstruction of particles in complex flow field environments with low computational cost. Meanwhile, to address the trade-off issues of reconstruction accuracy and smooth noise resistance in previous reconstruction algorithms, i.e., Tikhonov regularization, this paper employs dynamically adjusted regularization parameters in the LB-PINN algorithm. This approach ensures smooth noise-resistant processing while maintaining reconstruction accuracy, significantly reducing computation time and resource consumption. According to the experimental results, LB-PINNs demonstrate superior performance compared to previous reconstruction algorithms when reconstructing the soot volume fraction in complex reacting flow fields, i.e., combustion flame scenarios.

Thermal runaway evolution of a 4S4P lithium-ion battery pack induced by both overcharging and unilateral preheating

To clarify the thermal runaway characteristics of lithium-ion battery pack, this study has established a thermal runaway experimental platform based on actual power battery pack. A 4 in series and 4 in parallel battery pack was assembled using 86 Ah lithium iron phosphate batteries, and the experiment of thermal runaway induced by overcharging and unilateral preheating was carried out. The behavior and characteristics including the temperature change characteristics of each cell, the heat generated and transfer paths during thermal runaway propagation, the voltage changes of each serial module and the total voltage, flame evolution behavior, gas generation characteristics, debris, and mass loss were investigated. The research results show that module 1 was the first to experience thermal runaway due to preheating. The redistributed current caused the batteries in the remaining modules to rapidly generate heat. Subsequently, the heat transfer from module 1 triggered thermal runaway in modules 2, 3, and 4 in sequence. The entire flame combustion process lasted for 38 min, with the maximum temperature reaching 937.1 °C, resulting in thermal runaway in all batteries. The sequence of thermal runaway has been clarified, with the flame generated by the ignition of the battery casing, connecting tabs, and combustible gases emitted from the batteries serving as the primary paths for heat transfer and thermal radiation. The experimental results provide valuable insights into the thermal engineering issues of large-scale lithium-ion battery pack.

Experimental study on the burning rate and flame dynamics of oil reservoir pool fire under cross-wind: Effect of lip height

The study of combustion characteristics and heat feedback mechanisms of liquid fuels is important for fire safety engineering, particularly in the context of energy utilization. In thiswork, the effect of lip height on the evolution of n-heptane pool fire burning rates and flame heights, as well as the thermal feedback control mechanism, were investigated in a series of experiments in the presence of cross-wind. The results showed that the lip height of pool and cross-wind speed have a significant effect on the flame characteristics and heat feedback mechanism. For a given wind speed condition, the radiative feedback fraction and the convective feedback fraction showed non-monotonic changes with an increase in the lip height of pool. With the increase of lip height and cross-wind speed, the lift off phenomenon gradually appeared inside the oil pool, while the flame height outside the oil pool gradually decreased. A new correlation is proposed, which can well describle the relationship between the flame height and the ratio of the air entrainment caused by the cross airflow to that caused by the flame buoyancy, which can provide references for the utilization and management of energy storage strategies nowadays.

Effect of ignition timing on combustion characteristics and emissions of an X-type rotary engine with the high efficiency hybrid cycle

X-type rotary engines (XREs), using the high efficiency hybrid cycle, possess advantages of simple structure, high power-to-weight ratio, and high theoretical efficiency, making XREs as potential power sources in hybrid vehicles and small unmanned aerial vehicles. Typically, ignition timing has a promising potential to improve the combustion performance of spark ignition engines. In this work, the effects of ignition timing on flame propagation, combustion characteristics, and pollutants formation were studied adopting a three-dimensional simulation method. The results showed that at different engine speeds, significantly different flow forms and flow velocities in the larger chamber recess of the XRE were formed, resulting in large differences in flame propagation velocities at leading side and trailing side of the combustion chamber. Advancing ignition timing increased the flame propagation velocity at the given engine speed. Ignition timing had a more significant effect on peak pressure at medium-high speeds compared to low engine speeds. At 3000 RPM and 5000 RPM, the peak pressure was increased significantly with advancing ignition timing, but when ignition timing was advanced to -30 °CA, the increment in the in-housing pressure was minimal. As ignition timing was retarded, the ignition delay was slowly reduced. The effect of ignition timing on ignition delay was more significant for low speeds than for medium-high speeds. However, the influence of ignition timing on combustion duration showed the opposite trend. Delayed ignition showed a slight increase in indicated thermal efficiency and indicated mean effective pressure at low speeds, while the opposite was true at medium-high speeds. Nevertheless, the effect of ignition timing and engine speed on indicated specific fuel consumption is opposite to the trend of the above laws. With the increase of ignition timing, the NOx mass fraction first increased and then decreased at low speeds, with the maximum occurred at -30 °CA, but the NOx mass fraction continued to increase at high speeds. Soot and CO productions were slightly affected by ignition timing. To sum up, this XRE, setting the ignition timing of -30 °CA, achieved better flame propagation and engine performance at the price of a smaller increment of NOx emissions.

Investigation of swirl premixed dimethyl ether/methane flame stability and combustion characteristics in an industrial gas turbine combustor

In this paper, a new scheme for DME/methane (CH4) blended fuel combustion in an industrial gas turbine is proposed and numerical calculations are conducted to reveal the swirl premixed flame stability and combustion characteristics. The results show that injecting recirculated flue gas into the inlet gas restrains the flashback induced by high DME blending ratios and achieves the fully substitution of CH4 by DME. The DME/CH4 flame stability boundary is strongly correlaterd with the laminar flame speed and the adiabatic flame temperature. Therefore, it is recommended to set up the operating conditions of DME/CH4 industrial gas turbines by maintaining a fixed value of the laminar flame speed or adiabatic flame temperature to guarantee burner safety. Additionally, the combustion characteristics of DME/CH4 flames were evaluated under different operating conditions. The results indicate that the two uniformity factors PF and γT are largely independent of the DME blending ratio. As the equivalence ratio increases, PF shows a slight overall rise but remains below 1 %. NO and CO emissions are both directly proportional to the DME blending ratio and the equivalence ratio. Notably, as the flame transitions from a stable state to combustion-induced vortex breakdown flashback, the growth rates of NO and CO emissions rise sharply. In this case, NO emissions increase by 136 %, while CO emissions rise by 126 %.

High spatiotemporal resolution optical measurements of two-stage ignition and combustion in Engine Combustion Network Spray D flames

This study explores flame structures and combustion dynamics in high-pressure n-dodecane fuel sprays, focusing on the formation and consumption of formaldehyde (CH2O) during autoignition and the development of poly-aromatic hydrocarbons (PAH) as soot precursors. These processes are crucial for optimizing combustion efficiency and reducing emissions. However, traditional approaches, which rely on single-shot measurements or ensemble-averaged visualizations, often overlook critical early-stage processes during low-temperature ignition. To overcome these challenges, we employed an innovative high-speed planar laser-induced fluorescence (PLIF) technique at 50 kHz using a pulse-burst Nd:YAG laser system with an excitation wavelength of 355 nm. This approach, applied for the first time to Engine Combustion Network (ECN) Spray D flames, provides unprecedented insights into the combustion processes at varying ambient temperatures and oxygen concentrations. Additionally, simultaneous high-speed schlieren imaging at 100 kHz was used to visualize spray penetration, first-stage ignition, and thermal expansion zones. Our findings reveal that, similar to Spray A flames, CH2O forms in cold, fuel-rich zones well upstream of the combustion zone. However, in Spray D flames, the schlieren signal softening observed in the jet's head does not lead to complete disappearance, and the CH2O signal is absent from the full head of the spray. During the second-stage ignition, CH2O consumption accelerates due to high-temperature reactions, leading to a significant reduction in its signal. Unlike the mushroom-shaped structure seen in Spray A flames, Spray D flames exhibit a quasi-steady PAH phase structure, with lean peripheral mixtures insufficient for soot precursor formation. Notably, reducing ambient oxygen concentration to 13 % while maintaining or increasing temperature prolongs the presence of CH2O, highlighting its influence on ignition dynamics and oxidation processes in dodecane spray flames. This study provides new insights into the combustion mechanisms of high-pressure sprays and offers valuable data for developing next-generation combustion technologies, including models, aimed at improving efficiency and reducing emissions.